Is it overheating because it's losing coolant? It has to be going somewhere if it is.
If it's not losing coolant, then that narrows things down a bit. Most common causes are (1) coolant not circulating, which could be (a) radiator stopped up internally, (b) failed or failing water pump (includes slipping or failed belt), (c) closed thermostat (which I see yours has been removed), (d) collapsed lower radiator hose (this is the "suction" one), or (2) coolant is circulating, but heat is not being removed from it, which could be (e) radiator fins stopped up with dust, dirt, leaves, something blocking air flowing through them, or (f) fan isn't drawing air through radiator.
The 5.9 Cummins engines are generally pretty good, yours is an '89 model? It should be checked at some point for the Cummins KDP issue (there's a thread on it somewhere on this forum), if by some small chance the engine has been replaced, make sure it's not a so-called 53 block casting (produced late 90's-early 2000's), these had a problem from the foundry leading to the block cracking after some age. Now if it's losing coolant, an external leak should be traceable. An internal leak will go *Somewhere*, either into the oil (which you indicate is not the case), or burned out the exhaust, usually leaving white smoke (which you also say is not the case).
Also, you say an injector is spraying diesel and the corresponding cylinder is not getting compression? Without a chance to do a proper diagnosis it makes it hard to form an accurate conclusion ... is the injector spraying fuel outside the engine? If so, it will need to be replaced (or at least find/correct the cause), and it will cause a skip, which could lead to someone thinking the engine has a dead cylinder. Has the compression in that cylinder actually been tested? (Actually, if you're going to the trouble to do one, you might as well do the rest while you're at it).
Anyway, keep us posted, and gooooood luck.